University of California, San Diego (UCSD) researcher Zhaowei Liu and colleagues have taken the first steps in developing high-modulation-rate blue and green LEDs for underwater optical communications. They have created a nanostructured metamaterial with silver (Ag) and silicon (Si) that boosts the spontaneous-emission rate rate of a fluorescent light-emitting dye molecule—Rhodamine 6G (R6G)—by a factor of 76, as well as increasing the emission intensity of the dye by a factor of 80.1
The nanopatterned hyperbolic metamaterial (HMM) causes normally nonradiative plasmonic modes to radiate outward as light, resulting in the very large spontaneous-emission rate. In theory, the HMM used can be tuned to any wavelength in the visible spectrum for use with other dyes by altering the ratio of Ag to Si in the metamaterial.
To create the HMM, alternating layers of Ag and Si, with each layer about 10 nm thick, were deposited on a glass substrate, resulting in a total film thickness of 305 nm. Grating patterns with differing periods were then milled into the multilayer film using focused ion-beam milling, and R6G dye mixed with PMMA (a polymer) was spin-coated on top to a thickness of 80 nm.
The enhancement of the dye’s spontaneous-emission rate by the HMM was measured by measuring lifetimes of R6G molecules using time-resolved photoluminescence with a two-photon microscope, taking measurements on both grating-patterned and nonpatterned films. Both far-field emission and actual fluorescence intensity were enhanced by the HMM.
The 76-fold decay-rate enhancement and 80-fold emission enhancement for R6G was achieved using a grating with a period of 80 nm; this occurs because smaller grating periods better match high-wavevector plasmonic modes, resulting in better outcoupling (see figure). The researchers note that further-optimized grating designs could further boost light output by boosting the outcoupling of high-wavevector plasmonic modes.
Aiming at underwater-transmission wavelengths
“The major purpose of this program is to develop a better light source for communication purposes,” says Liu. “But this is just a first step in the whole story. We have proved that this artificial, man-made material can be designed to enhance light emission and intensity, but the next step will be to apply this on conventional LEDs.”
Extremely high modulation rates in gallium nitride (GaN)-based blue- and green-emitting LEDs is a missing link that is necessary for increasing the rate at which information can be sent via optical channels underwater, such as between ships and submarines, submarines and divers, underwater environmental sensors and unmanned underwater vehicles, or other combinations (water transmits best in blue and green).
Currently, the modulation rate for GaN-based LEDs is less than 1 GHz, a rate slower than the speed of most WiFi signals, says Liu.
The Rhodamine molecule used in the UCSD experiment gives off a yellow-green hue. The next step for the researchers will be to pair the nanostructured metamaterial with GaN-based LEDs.
“The design of the materials may not be the hardest thing,” said UCSD graduate student Dylan Lu, who noted that the metamaterial will work with LEDs that have been manufactured to a specific industry standard. “I think the major challenge, to apply it to LEDs, will be an integration issue.”
Liu recently won a grant from the Office of Naval Research (Arlington, VA) to develop the high-modulation-rate blue and green LED systems; the grant totals a little more than $500,000 over three years.
REFERENCE
1. D. Lu et al., Nat. Nanotechnol., doi:10.1038/nnano.2013.276 (2014).
